This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Protein/RNA interactions in the life cycle of the HIV virion involve complex interactions in a heterogeneous environment. Electron Spin Resonance (ESR) is a powerful experimental tool to study protein/RNA interactions when the interacting species have limited intrinsic structure. Multifrequency ESR is particularly sensitive to details of rotational dynamics on the microsecond to nanosecond timescale, where many biochemically significant processes occur. On the basis of many previous studies and careful numerical algorithm development, it is known how to extract reliable, informative results from multifrequency ESR spectra. We propose to study a model system of spin-labeled TAR-RNA interacting with the protein NCp7 that would be difficult to study by other means due to its flexible structure. In particular, we intend to use multifrequency ESR to study dynamic and structural aspects of hairpin oligonucleotide-nucleocapsid protein folding and recognition. Promising initial results have been obtained at 240 GHz on spin-labeled TAR-RNA with associated NCp7 in a high salt environment.